Derbi drd 125 exhaust inlet

Hi there,
The true top speed of the Derbi Senda DRD 125 R/SM is actually supposed to be 77mph, having the same bike but the SM model I can say confidently it doesn't go that fast unless it is my weight!

If the lad you have bought it off put in a 115mm jet into the carb this could be a reason why you are lacking speed
People believe by upjetting their carb it makes the bike go faster but this isn't always true, you usually upjet to cater for other modifications such as big bore kit, sports exhaust or performance air filter.
Personally I would change the jet back to what it should be and try from there, unless you wanted to spend countless hours fiddling with the air/fuel mixture trying to gain the right amount of air for the size of jet

Hope this helps